My only problem with that interpretation is that Harapior warned her not to try and cant with the Collar on after she'd proved torture didn't work on her, after all the singing and the other Nonmen saying that she tormented him. He still seemed to think the Collar would kill her if she used sorcery, even though he absolutely knew that pain had no hold over her. To me, that implies that merely being able to ignore pain shouldn't normally protect against an Agonic Collar, and that Serwa had to do something else (that Harapior could not conceive of) to defeat it.
I think Harapior was just hedging his bets. He gagged her because he knew her as Dunyain and so knew she sang the lays of the Nonmen so that Nin'ciljiras would bid her sing for the court before they defiled her. Harapior seems to be the only Ishterebinth Nonmen who has a clue as to how dangerous the Dunyain can be so he assumes that her signing to torment the torturers has a point - that she can sing through the Agonic Collar and thus when bid sing for Nin'ciljiras, she would settle the issue of civil war.
I think he specifically says it's an Agonic Collar made by Emildilis but since he still gags her on top of the Agonic Collar, he probably is just trying to scare her out of trying.
It's a shame her showing her father's dread portion happens off screen.
Damn shame but also classic Bakker. Leaves the imaginings up to us
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EDIT: Fixed boneheaded error.
